You may know when you're awake, but you can't prove it scientifically. That's because there's no objective way to distinguish between a "conscious" and "unconscious" brain. But now, scientists may have discovered the telltale neurological signature of a mind that's awake.

Over at the New York Times, Maggie Koerth-Baker has a great essay on the scientific quest to discover what consciousness looks like in the brain. Interestingly, our greatest insights have come from studying what unconsciousness looks like. She begins:

More than a decade ago, a 43-year-old woman went to a surgeon for a hysterectomy. She was put under, and everything seemed to be going according to plan, until, for a horrible interval, her anesthesia stopped working. She couldn't open her eyes or move her fingers. She tried to breathe, but even that most basic reflex didn't seem to work; a tube was lodged in her throat. She was awake and aware on the operating table, but frozen and unable to tell anyone what was happening.

Studies of anesthesia awareness are full of such horror stories, because administering anesthesia is a tightrope walk. Too much can kill. But too little can leave a patient aware of the procedure and unable to communicate that awareness. For every 1,000 people who undergo general anesthesia, there will be one or two who are not as unconscious as they seem — people who remember their doctors talking, and who are aware of the surgeon's knife, even while their bodies remain catatonic and passive. For the unlucky 0.13 percent for whom anesthesia goes awry, there's not really a good preventive. That's because successful anesthetization requires complete unconsciousness, and consciousness isn't something we can measure.

There are tools that anesthesiologists use to get a pretty good idea of how well their drugs are working, but these systems are imperfect. For most patients receiving inhaled anesthesia, they're no better at spotting awareness than dosing metrics developed half a century ago, says George Mashour, a professor of anesthesiology at the University of Michigan Medical School. There are two intertwined mysteries at work, Mashour told me: First, we don't totally understand how anesthetics work, at least not on a neurological basis. Second, we really don't understand consciousness — how the brain creates it, or even what, exactly, it is.

Currently anesthesiologists look at our brain waves to measure wakefulness. But new experiments show that the true sign of a wakeful person may be the way signals travel around inside our brains — not which signals are emanated by them. The implications are fascinating. They reveal how sleeping minds work, as well as what it means to be fully conscious. Plus, today's neuroscientists may actually have discovered concrete evidence that proves the theories of consciousness first advanced by nineteenth century philosopher Emmanuel Kant.